The U.S. and European allies have concluded that Syria has used chemical weapons against rebels "multiple times," and the White House said it would provide additional assistance to Syrian opposition that "includes military support."

In a conference call Thursday afternoon, White House Deputy National Security Adviser for Communications Ben Rhodes said that the "intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date."

Obama "has said that the use of chemical weapons would change his calculus, and it has," Rhodes said.

For now, though, the White House was unspecific about the next steps it would take,

"We've prepared for many contingencies in Syria," Rhodes said. "We are going to make decisions on further actions on our own timeline."

Rhodes said that Russia has been notified of the chemical weapons findings but has not yet committed to having Syrian President Bashar al-Assad step down. He added that "several allies" and the United Nations have also been briefed.

The Wall Street Journal reported that military planners have proposed a limited no-fly zone that stretches up to 25 miles into Syria, but the White House said it has not yet made a decision.

There is "no clear guarantee" that it would help, Rhodes said.

Brendan Buck, a spokesman for House Speaker John Boehner, urged Obama to consult with Congress on further steps.

"It is long past time to bring the Assad regime’s bloodshed in Syria to an end. As President Obama examines his options, it is our hope he will properly consult with Congress before taking any action," Buck said.